1. Field of the Invention
The present invention relates to an exhaust gas purification system of an internal combustion engine having a particulate filter for collecting particulate matters included in exhaust gas, which is discharged from an exhaust port of the engine. Specifically, the present invention relates to regeneration of the particulate filter.
2. Description of the Related Art
Conventionally, an exhaust gas purification system for reducing air pollutants discharged from an exhaust port of an internal combustion engine is introduced as one of measures for protecting the environment. As an example of the above exhaust gas purification system, a catalyst system employing an oxidation catalyst, a nitrogen oxide catalyst, a three-way catalyst or the like is publicly known. In recent years, particulate matters such as soot or a soluble organic fraction (SOF) included in exhaust gas of a diesel engine have been problems. As one of countermeasures against the particulate matters, an exhaust gas purification system having a particulate filter in an exhaust passage is employed. If the exhaust gas enters the particulate filter, the exhaust gas passes through porous partition walls. At that time, the particulate matters are collected at surfaces or pores of the partition walls. If a quantity of the collected and accumulated particulate matters increases excessively, a fluid resistance in the particulate filter increases and a back pressure of the internal combustion engine increases. As a result, output of the engine will be reduced, for instance. Therefore, the particulate filter is regenerated by combusting and eliminating the accumulated particulate matters regularly.
An exhaust gas purification system disclosed in U.S. Pat. No. 5,211,010 (Patent Document 1) estimates the quantity of the accumulated particulate matters and determines regeneration timing of the particulate filter based on the result of the estimation. The accumulated quantity is calculated from a pressure difference across the particulate filter or a flow rate of the exhaust gas, based on the fact that a pressure loss at the particulate filter is increased by the accumulation of the particulate matters. If the quantity of the accumulated particulate matters exceeds a threshold value, it is determined that regeneration start timing is reached.
Ashes enter the particulate filter with the particulate matters. The ashes are incombustible particles, which are produced mainly through reaction between calcium included in engine oil and sulfur included in fuel. The ashes entering the particulate filter are prone to be collected by the particulate matters.
A particle diameter of the ash ranges from 0.1 micrometer to several micrometers when the ash is produced in a combustion chamber and is sufficiently smaller than an average diameter of the pores of the particulate filter, which ranges from ten micrometers to several tens of micrometers. If the particle diameter of the ash is in the above range (0.1 micrometer to several micrometers), the ash passes through the pores during the regeneration of the particulate filter, in which the particulate matters are combusted and eliminated, and is discharged from the particulate filter. However, the particle diameter of the ash grows while the ash stays within the particulate filter. Therefore, in some growing state of the particle diameter of the ash, the ash becomes less prone to be discharged from the particulate filter even when the particulate filter is regenerated. In this case, there is a possibility that the pressure loss at the particulate filter cannot return to the original level because of the remaining ashes even if the particulate matters are combusted and eliminated.
Even the technology disclosed in Patent Document 1, which combusts and eliminates the accumulated particulate matters regularly, cannot sufficiently prevent the ashes from remaining in the particulate filter. More specifically, if a heavy-load operating condition continues, the particulate matters accumulated in the particulate filter are combusted and eliminated not through the regeneration control because the temperature of the particulate filter becomes high under the heavy-load operating condition. In this case, there is a possibility that the eliminated quantity of the particulate matters is balanced with a quantity of the particulate matters newly entering the particulate filter, so the quantity of the accumulated particulate matters does not increase. In this case, it is not determined that the start timing of the regeneration is reached, so the ashes will stay in the particulate filter for a long time. As a result, the particle diameter of the ash will grow further.